The present invention is generally related to a photoresist barrier layer capable of reproducing fine details and more particularly related to a barrier layer in an inkjet printer printhead that utilizes small dimensions to produce reduced drop weight ink drops.
Inkjet printers operate by expelling a small volume of ink through a plurality of small orifices in an orifice plate held in proximity to a medium upon which printing or recording marks are to be placed. These orifices are arranged in a fashion in the orifice plate such that the expulsion of drops of ink from a selected number of orifices relative to a particular position of the medium results in the production of a portion of a desired character or image. Controlled repositioning of the orifice plate or the medium followed by another expulsion of ink drops results in the creation of more segments of the desired character or image. Furthermore, inks of various colors may be coupled to individual arrangements of orifices so that selected firing of the orifices can produce a multicolored image by the inkjet printer.
Several mechanisms have been employed to create the force necessary to expel an ink drop from a printhead, among which are thermal, piezoelectric, and electrostatic mechanisms. While the following specification is made with reference to a thermal ink ejection mechanism, the present invention may have application for the other ink ejection mechanisms as well.
Expulsion of the ink drop in a conventional thermal inkjet printer is a result of rapid thermal heating of the ink to a temperature that exceeds the boiling point of the ink vehicle to create a vapor phase bubble of ink. Such rapid heating of the ink is generally achieved by passing a pulse of electric current through an ink ejector that usually is an individually addressable heater resistor, typically for 1 to 3 microseconds, and the heat generated thereby is coupled to a small volume of ink held in an enclosed area associated with the heater resistor and that is generally referred to as a firing chamber. For a printhead, there are a plurality of heater resistors and associated firing chambers--perhaps numbering in the hundreds--each of which can be uniquely addressed and caused to eject ink upon command by the printer. The heater resistors are deposited in a semiconductor substrate and are electrically connected to external circuitry by way of metalization deposited on the semiconductor substrate. Further, the heater resistors and metalization may be protected from chemical attack and mechanical abrasion by one or more layers of passivation. Additional description of basic printhead structure may be found in "The Second-Generation Thermal InkJet Structure" by Ronald Askeland et al. in The Hewlett-Packard Journal, August 1988, pp. 28-31. Thus, one of the walls of each firing chamber consists of the semiconductor substrate (and typically one firing resistor). Another of the walls of the firing chamber, disposed opposite the semiconductor substrate in one common implementation, is formed by the orifice plate. Generally, each of the orifices in this orifice plate is arranged in relation to a heater resistor in a manner that enables ink to be expelled from the orifice. As the ink vapor bubble nucleates at the surface of the heater resistor and expands, it displaces a volume of ink that forces a volume of ink out of the orifice for deposition on the medium. The bubble then collapses and the displaced volume of ink is replenished from a larger ink reservoir by way of an ink feed channel in another of the walls of the firing chamber.
As users of inkjet printers have begun to desire finer detail in the printed output from a printer--especially in color output--the technology has been pushed into smaller drops of ink to achieve the finer detail. Smaller ink drops means lowered drop weight and lowered drop volume. Production of such low drop weight ink drops requires smaller structures in the printhead. Thus, smaller firing chambers (containing a smaller volume of ink), smaller ink ejectors, and smaller orifice bore diameters are required.
A majority of the size of the firing chamber is determined by a layer of photoimagable polymer sandwiched between the heater resistor-bearing semiconductor substrate and the orifice plate. This layer is traditionally known as a barrier layer and has often been described, see for example, "Development of High-Resolution Thermal Inkjet Printhead", by William A. Buskirk, et al., the Hewlett-Packard Journal, October 1988, pp. 55-61. The barrier layer is honeycombed with cavities that, when bounded by the substrate on one side and by the orifice plate on the other, become the ink firing chambers and connecting inkfeed channels that route ink into the ink firing chambers. The dimensions and architecture of the ink firing chambers, the ink feed channels, and other features which control and filter ink are typically defined and created by photoimaging techniques. These techniques are capable of creating relatively small features in the barrier material.
A problem that occasionally manifest itself in inkjet printheads is that of occlusion or narrowing occurring in an ink feed channel or in the orifice of the printhead. Microscopic particles can become lodged in the channel leading to the ink firing chamber, causing premature failure of the heater resistor, misdirection of ink drops, or diminished ink supply to the firing chamber resulting in greatly diminished ink drop size. A single orifice, which does not fire an ink drop when it is commanded to do so, leaves a missing portion from a printed character or creates a band of missing drops from a printed image. The end result is perceived as a poorer quality of printed matter, a highly undesirable characteristic for an inkjet printer. To resolve this undesirable result, others have used spare or redundant orifices to eject ink, multiple inlets to the ink firing chamber, and pillars or islands formed in the barrier layer and disposed in the ink path to filter particles from the ink.
As the size of the firing chamber, ink feed channels, and filtering features become smaller--for example approximately the same dimensions as the thickness of the barrier layer--the conventional barrier layer photoimaging process is unable to resolve the finer details of the desired architecture. This inability places a limitation on the smallest size of the architectural features and will limit the reliability of the ejection of ink when the smallest features are not properly formed.